Electrolytic marking medium



United States Patent 3,168,050 ELECTRQLYTIC MARKING MEDIUM Gerald Mandel and Irving Liehlich, Eimhurst, N.Y., aslsggnfrs to Hogan Faximile Corporation, New York,

5 No Drawing. Filed Aug. 22, 1960, Ser. No. 51,202 8 (Ilairns. (Cl. 204-2) This invention relates to the art of electro-sensitive recording media and relates particularly to an improved recording medium employing a new marking compound. The invention is concerned especially with a novelelectrolytic recording method which does not involve or depend upon so-called lake-forming marking compounds and which does not involve or depend upon the formation of a color lake on the recording medium in order to obtain marking of the medium.

Heretofore, certain polyhydroxyphenolic compounds such as pyrocatechin (i.e., catechol) have been discovered to be capable of serving as marking compounds in aqueous electrolytic recording papers such as disclosed in US. Patent No. 2,339,267 to I. V. L. Hogan et a1. and US. Patent No. 2,358,839 to E R. Wagner. Such papers may be employed in electrical recording apparatus such as described in US. Patent No. 2,575,959 to J. V. L. Hogan. The method and principle of operation, by which all of the aforementioned polyhydroxyphenolic compounds and their derivatives serve to produce a mark on the recording medium, is described in these patents as involving the formation of a color lake on the medium which constitutes the mark. The aforesaid polyhydroxyphenolic compounds and derivatives are disclosed as being selected for their lake-forming characteristic.

Recording paper of the electrolytic type is generally marked by disposing the paper between a positive metal anode electrode and a negative cathode electrode. The paper is impregnated with an electrolytic solution containing as one ingredient a marking compound. Speaking generally, when a voltage is applied between the electrodes and current flows through the paper, the marking compound is capable of reacting with ions supplied by the anode to form a colored mark on the surface of the paper wherever the anode contacts the paper adjacent to the cathode disposed on the other side of the paper.

In order to prove acceptable for various facsimile and related recording purposes, the recording paper must meet cetain important requirements. The paper is preferably white initially and must have the ability to produce marks having a density which is a function, preferably linear, of the current passed through the paper. The color tone obtained should be able to vary uniformly in a range from white (ie., for zero current flow) through grays (ie., for currents of intermediate magnitudes) to black (i.e., for high or maximum current magnitude). Marks having a substantially purple, green or brown cast are generally not desirable. There also should be a minimum of bleeding or fringing of the mark so as to maintain sharp detail for the mark. The wet paper, while stored in a special container for extended periods of time prior to use, must remain stable (fresh) without chemical decomposition of its ingredients which would cause discoloration of the paper prior to use and would leave a discolored background after recording.

Moreover, it is very important to employ a recording paper capable of yielding a recorded copy which will remain stable to light and will not streak either during or after recording and after exposure to light for an extended period of time. Similarly, it is desirable to obtain a recorded copy which will remain insensitive to light and will not discolor after standing or even after being repeatedly run through any of the conventional duplicating machines involving exposure to ultra-violet light, such as an Ozalid machine.

Another very important consideration is the provision of a recording paper wherein the marking compound or other active ingredients in the paper will not diffuse or transfer, during or after recording, to adjacent sheets or surfaces of marked or unmarked paper nor to adjacent areas such as the walls of the recorder cabinet or of the room itself. Such transfer frequently results in an objectionable discoloration of the adjacent paper surfaces and walls. a

In certain applications difficulty has been experienced in the past with the use of recording papers employing pyrocatechin (catechol) as the marking compound. An especially troublesome problem in the use of catechol recording papers is the tendency for the catechol to diffuse or transfer to, and to discolor, adjacent sheets of paper and other surfaces adjacent to the recorder during and after recording. In order to obtain marks of full density when using the catechol recording papers disclosed in the aforementioned patents, it has generally been found necessary to provide an electrical heater device adjacent to the paper after it passes from between the electrodes. In the presence of heat the undesirable transfer characteristics of catechol are still further increased.

It is therefore an object of the invention to provide improved electro-responsive graphic recording media.

It is another object to provide electro-sensitive recording media containing as a marking compound a compound which heretofore has not been employed for this purpose.

It is another object to provide a new electrolytic recording method and media which does not depend upon so called lake-forming compounds nor the formation of color lakes in order to mark the media.

It is another object to provide new electro-sensitive recording media wherein there is no external diffusion or transfer of the ingredients of the media during or after recording.

A further object is to provide a White electrolytic recording medium which does not contain catechol but rather contains a new substantially colorless marking compound which, both before and after recording, does not discolor or otherwise alter the white appearance of the medium.

Another object is to provide an electrolytic recording medium which contains a marking compound other than catechol and yet still provides a mark having a color tone which includes a range of gray colors up to and including dense black rather than providing a mark which is more purple, red or brown than it is black.

It is a further object to provide electrolytic recording media which is chemically stable prior to use and which will: remain stable to light both during and after recording and which will permit the recorded copy to be exposed repeatedly to ultra-violet light in a duplicating machine.

It is an especially important object to provide electrolytic recording media with new marking compounds which are capable of being used with any bromide or chloride as the electrolyte without the medium becoming unstable to light (including ultra-violet light) or streaking during or after recording as heretofore experienced.

Other objects and advantages of the invention will be apparent to those skilled in the art.

In a co-pending application Serial No. 4629, now Patent No. 3,024,173, filed January 26, 1960, by G. Mandel, one of the co-inventors herein, protocatechuic acid is disclosed as a marking compound and as accomplishing the foregoing objects.

In accordance with the present invention we have discovered that polyhydroxyphenolics with at least one conjugated side chain acid constituent and having the general formula where rz=l, 2, 3, and where K, R R may be alkyl or aryl groups, NH OR or H; where R is an alkyl or aryl group or H; and where R and R may be H, any alkyl or aryl group, and may be substituted for the protocatechuic acid of said prior application. Hereinafter for convenience in terminology we will refer to the above stated class as the caffeic acid group, since caffeic acid (3,4 dihydroxycinnamic acid) is the parent compound of the class. The above formula is caffeic acid when R R R R and R each is H, and 11:1, i.e.

I CH=CHC O OH In using caffeic acid as a marking compound, as in using protocatechuic acid, any bromide or chloride may be used as the electrolyte without the recorded print having streaks therein. In this connection it should be noted that while certain prior patents, such as US. Patent No. 2,339,267 supra, suggest that various chlorides and bromides may be used as the electrolyte in place of a nitrate, nevertheless subsequent experience through the years has revealed to the art that no chloride or bromide could be successfully employed as the electrolyte in these electrolytic recording media. The reason is that the recorded copy obtained from paper containing any chloride or bromide as the electrolyte, in combination with any specific marking compound hitherto employed, was invariably found to become streaked during and after recording. An exception is the recording paper impregnated according to said G. Mandel application. Consequently, to avoid streaking it has been the general practice to utilize a nitrate, such as potassium nitrate, as the electrolyte in recording papers. Prior recording papers containing the various nitrates as electrolytes are unfortunately lightsensitive in that they discolor after exposure to light, particularly ultra-violet rays, during and arter recording, such as when the recorded copy is stored or run through an Ozalid duplicator machine or the like.

In accordance with the invention there is provided an electrolytic recording medium, in which the recording medium comprises an inert, porous and preferably white support, such as a sheet of paper having a high wetstrength, which is impregnated with a solution of a member of the caifeic acid group and an electrolyte. The electrolyte may be a nitrate but preferably is a chloride or a bromide. The impregnant may also include a stabilizer, an acidifier, and may include one or more anti-fringing agents.

Recording paper may be impregnated with any of the following formulations which are given by way of example:

Example N 0. 1

Sodium chloride grams 400 Caffeic acid do 70 Thiourea do Water cc 4000 4 Example N0. 2 Sodium chloride grams 400 Citric acid do 10 Catfeic acid do 70 Thiourea do 20 Monosodium phosphate do 4 Water cc 4000 1 pH will be approx, 2.4.

Example N0. 3 Potassium nitrate grams 10 Caifeic acid do 2 Thiourea =do 0.5 Water cc In the above examples, the quantities given for the caffeic acid represent the aproximate solubility limit for this material.

Any member of the caffeic acid group may be substituted in the above examples for caifeic acid such as, for example, as those in which R R, R are methyl groups rather than hydrogen atoms.

In Examples No. 1 and No. 2, the sodium chloride electrolyte may vary from zero to about 1200 grams (the solubility limit) depending upon the degree of electrical conductivity desired, and in Example No. 3 the potassium nitrate electrolyte may similarly vary from zero to 35 grams. In Examples N0. 1 and N0. 2 the sodium chloride may be replaced, without obtaining streaking, by sodium bromide or by any of the chlorides or bromides of any metal the ions of which do not react with the other components in the solution. Typically, these metals include calcium, potassium, magnesium, cesium, lithium or rubidium. In Exmnple No. 3 the potassium nitrate may be replaced by any of the above chlorides or bromides, and for certain applications where light sensitivity may be tolerated, the potassium nitrate may be replaced by nitrates of the above metals. Similarly in Examples No. 1 and No. 2 the sodium chloride may be replaced by nitrate sof the above metals for those applications where light sensitivity may be permissible.

In the above examples the thiourea serves as a stabilizer and may be replaced by another suitable stabilizer such as one of the alkyl derivatives of thiourea or dithiobiuret. In Examples No. 1 and No. 2 the thiourea may be varied approximately from 5 grams, below which instability occurs, up to about 360 grams which is the approximate solubility limit for the thiourea. Similarly in Example No. 3, the thiourea may vary aproximately from 0.1 gram up to 9.0 grams.

In Example No. 2 the citric acid serves an anti-fringing agent and also as an acidifier. The citric acid may vary preferably from zero to about 50 grams, above which the density of the mark becomes appreciably less. Citric acid may be employed in Example No. 1 in the same quantity and range. In Example No. 3 an acid such as citric acid or sulfuric acid may be used as an acidificr. In the above examples the pH preferably is in the range from about 1.0 to 3.5. With a pH value above 3.5 the mark is apt-to change color. The pH may be less than 1.0 in those applications where a lighter mark is aceptable. In Example No. 2 the monosodium phosphate provides an aditional anti-fringing function. It may be varied from zero to about 20 grams, above which the density of the mark becomes appreciably less.

Recording paper may be prepared by passing the porous white paper sheet or other porous support of high wet-strength through a bath prepared in accordance with one or" the three above formulae given as examples. Approximately 2.5 to 3.0 cc. of the solution is required per square foot of paper in a typical case. The solution is permitted to distribute itself evenly throughout the paper. The excess impregnant may be removed, as by passing the paper through pressure rolls, so that when ready for use in a recorder machine, the recording paper will have about 40% moisture by weight. In order to retard evaporation, the recording paper is stored in sealed containers until needed.

During the recording process a recorder, such as the type referred to above, may be employed in which an electric current is passed through the recording paper sandwiched between cathode and anode electrodes in contact with opposite surfaces of the paper. The anode may be composed of iron, stainless steel or some other eroding metal capable of reacting with the cafieic acid in the solution to form colored complex compounds which constitute the recorded color marks on the paper or other support. The recorded paper may then be dried in the air or by some heating device, and the recorded marks will be permanently set. Although not required in order to obtain adequate density of the marks on recording media in accordance with the invention, nevertheless a special heater such as disclosed in U.S. Patent No. 2,485,- 678 may be employed with the recorder if desired.

Certain specific combinations, materials and specifications have been given by way of example. Various other modifications may be made. For example, a formulation containing cafieic acid as the principal marking compound may also contain, in accordance with the present invention, one or more other marking compounds in addition to obtain any further improvement desired. It will now be apparent to those skilled in the art that various changes and modifications may be made within the spirit and scope of the invention as defined by the appended claims.

We claim:

1. An electrolytic recording medium comprising an impregnated sheet containing as a marking compound caffeic acid in an electrolytically conducting solution.

2. An electrolytic recording medium according to claim 1 in which the pH of the impregnant is in the range of 1.0 to 4.0.

3. An electrolytic recording medium according to claim 1 in which the solution contains at least one compound selected from the group consisting of sulfuric acid, nitric acid, hydrochloric acid, citric acid and salts thereof, oxalic acid and salts thereof, and formic acid.

4. An electrolytic recording medium according to claim 1 in which a stabilizer is incorporated against degradation in the moist condition prior to marking.

5. An electrolytic recording medium according to claim 1 in which a stabilizer is incorporated and is selected from the group consisting of thiourea, alkyl derivatives of thiourea, and dithioubiuret.

6; An electrolytio recording medium according to claim 1 in which the electrolyte is selected "from the group consisting of chlorides and bromides of sodium, calcium, potassium, lithium, magnesium, rubidium, and cesium.

7. An electrolytic recording medium according to claim 1 in which is included a stabilizer, the impregnant consisting essentially in approximate parts by relative weight of marking compound 7, electrolyte 40, and said stabilizer 2.

8. An electrolytic recording medium according to claim 1 in which is included a stabilizer, the impregnant consisting essentially in approximate parts by relative weight of caffeic acid 7, sodium chloride 40, citric acid 1, and said stabilizer being thiourea 2.

References Cited in the file of this patent UNITED STATES PATENTS Richards Aug. 16, 1960 Mandel Mar. 6, 1962 OTHER REFERENCES 

1. AN ELECTROLYTIC RECORDING MEDIUM COMPRISING AN IMPREGNATED SHEET CONTAINING AS A MARKING COMPOUND CAFFEIC ACID IN AN ELECTROLYTICALY CONDUCTING SOLUTION. 